Simon is having trouble using linked workbooks, so he consults his colleague Brooke for help. Simon tells Brooke that when he attempts to open a workbook, the Edit Links dialog box appears with the message "This workbook contains one or more links that cannot be updated." Brooke suggests several possible causes of this error message. Which of the following is a possible cause?
a. the workbook was sent to him without links
b. one of the workbook's source files was edited
c. one of the workbook's source files was copied to a different folder
d. one of the workbook's source files was replaced with an updated file of the same name